1. Field of the Invention
The present invention relates to a micro-electromechanical structure, in particular to an accelerometer, with improved insensitivity to the thermomechanical stresses induced by the package.
2. Description of the Related Art
As is known, micromachining techniques enable manufacturing of micro-electromechanical structures (MEMS) within layers of semiconductor material, which have been deposited (for example, a polycrystalline silicon layer) or grown (for example, an epitaxial layer) on top of sacrificial layers, which are removed via chemical etching.
In particular, accelerometers obtained using micromachining techniques comprise mobile regions (rotor regions) suspended with respect to a substrate, and fixed regions (stator regions) fixed to the substrate and in particular to the package of the accelerometer. The rotor regions are connected to the substrate through elastic biasing elements (called springs), and are mobile with respect to the stator regions along one or more axes, which constitute the axes of detection of the accelerometer.
The various regions that make up the micro-electromechanical structures can have different coefficients of thermal dilatation, especially when they are subjected to different dopings, and consequently residual thermomechanical stresses may be present in the microstructures at the end of machining. In particular, the material of the package has a different coefficient of thermal dilatation as compared to the material of the micro-electromechanical structure (generally monocrystalline or polycrystalline silicon). Consequently, the suspended masses can be subjected to minor relative displacements with respect to the fixed regions of the micro-electromechanical structure.
The presence of residual stresses leads to considerable problems for the proper operation of the micro-electromechanical devices. For example, in the case of micro-electromechanical structures comprising a mobile mass having a plurality of anchoring points, the thermomechanical stresses, acting in a different and non-uniform way on the various anchoring points, tend to create tensile and compressive stresses and to modify the positions of the various parts of the structure with respect to one another. This leads to alterations in the performance of the devices, in particular measurement errors and drifts, which moreover vary according to the production lot, and at times also between devices belonging to a same production lot.
In order to overcome these problems, U.S. Patent No. 6,508,124 in the name of the present applicant and published in the U.S. on January 21, 2003, describes a micro-electromechanical structure comprising a rotor having a centroidal axis and a suspended mass arranged at a certain distance from the centroidal axis. A plurality of mobile electrodes extend from the suspended mass. The rotor is supported and biased by a suspension structure having a single central anchoring portion extending along the centroidal axis and integral with a body of semiconductor material. The micro-electromechanical structure further comprises a stator made up of a plurality of fixed electrodes, comb-fingered with respect to the mobile electrodes, each fixed electrode being fixed to a respective anchorage. The fixed electrodes and the respective anchorages are arranged at least partially inside the suspended mass, some of them being close to the rotor anchorage.
The presence of a single central anchoring region for the rotor, arranged along the centroidal axis of the structure, enables an improved insensitivity to thermomechanical stresses to be obtained as compared to microstructures that have a plurality of anchoring elements for the rotor. In fact, in this second case, each anchoring element can undergo, on account of the thermomechanical stresses introduced by the package, a displacement different from the other anchoring elements, causing the deformation of the entire structure, a problem which, instead, does not exist with a single central anchorage.
The solution proposed in the aforementioned U.S. patent does not, however, enable a complete solution of the problems linked thermomechanical stresses, in so far as the rotor anchorage can in any case undergo displacements with respect to at least some of the stator anchorages. Since the measurement of acceleration is linked to the relative distance between rotor electrodes and stator electrodes, these displacements can lead to measurement errors and drifts.
U.S. Pat. No. 6,892,576 describes an accelerometer structure aimed at reducing the offsets due to the aforementioned thermomechanical stresses. In detail, the rotor is here formed by a suspended mass having a peripheral region defining a cavity, and by a plurality of elongated electrodes extending within the cavity from the suspended mass, while the stator is formed by a plurality of fixed electrodes comb-fingered with respect to the mobile electrodes. The suspended mass is anchored to the substrate by at least one anchorage arranged close to the center of the suspended mass. For anchoring the fixed electrodes to the substrate, instead, this solution provides a plurality of anchorages, one for each fixed electrode, of which one is arranged in the proximity of the rotor anchorage and the others are arranged gradually at an increasingly greater distance from the center of the suspended mass. A configuration of this type has the purpose of bringing the stator anchorages and rotor anchorages closer together so that they will undergo substantially equivalent movements in presence of thermomechanical stresses. However, the presence of numerous stator anchorages, and the consequent need to make the corresponding electrical biasing connections, renders it, however, in effect impossible to minimize the distance between all the rotor and stator anchorages, and to position the same stator-anchorages in a position effectively close to the center of the suspended mass.